Mechanistic Design Principles of Metal/Sodium-Salt Composites as Cathodes for Sodium-Ion Batteries

Better batteries are needed for electric transportation, grid energy storage, and a more secure energy supply. Sodium-ion batteries are attractive because sodium is abundant, inexpensive, and broadly available. However, many current sodium-ion cathodes still involve tradeoffs among energy, cost, and lifetime. This project explores a different path. Instead of relying on crystalline materials, it will use a carefully designed mixture of common materials. The cathode will be made by milling together a metal such as iron, a sodium salt, and a mediator material that helps charge move through the mixture. In this mixture, structural disorder and internal contact regions will create chemical environments that allow metals such as iron to release charge more effectively. The project will discover how these tiny contact regions control energy, power, and lifetime. The project outcomes will be design rules for selecting the metal, salt, mediator, and processing conditions needed to make low-cost sodium-ion cathodes from widely available materials. The project will also support education and workforce development. It will provide research opportunities for students at several career stages, including community college students, undergraduate researchers, and high school students in summer research programs. This project will establish mechanistic design principles for sodium-salt composite cathodes. These cathodes will contain three functional components: a transition metal that serves as the redox-active electron source, a sodium salt that supplies sodium ions and charge-compensating anions, and a mediator compound that promotes ion transport and reversibility. Preliminary studies show that ball-milled mixtures of sodium sulfate, iron, and copper sulfide can deliver far higher capacity than the binary sodium sulfate/iron mixture, indicating that mediator-enabled heterointerfaces play a central role in activating iron redox and reducing polarization. The project will determine the operating mechanism of the sodium sulfate/iron/copper sulfide model composite. The project will identify the charged and discharged products, measure the oxidation states of iron, copper, and sulfur species, and determine whether sulfate anions participate in local charge compensation. Comprehensive characterization will be conducted to track local structure, bonding, and valence changes across the state of charge. The project will connect heterointerface structures with ion transport, voltage hysteresis, and reversibility. This project will establish component-level design rules, which elucidate how each component controls operating voltage, capacity, hysteresis, and long-term cycling. Structural disorder will be treated as a measurable design variable rather than an uncontrolled defect. The contribution will be a predictive framework that links local coordination, interfacial properties, and mediator chemistry with electrochemical performance. This framework will guide the design of a broader family of low-cost, earth-abundant sodium-ion cathodes based on metal-salt composite materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.